The tearful admission represented an abrupt turn in a case where Mr. Kruger was accused of soliciting and taking at least $1 million in illicit payments, which prosecutors said subsidized a life of luxury that included a Bentley and an expansive mansion in Brooklyn. After he was indicted in March, Mr. Kruger forcefully denied the charges and pleaded not guilty.

On Tuesday, Mr. Kruger was emotional as he struggled through a prepared statement in Manhattan Federal Court. "My actions were in violation of the law, and I knew that they were wrong," he said. "I accept responsibility for my actions and am truly sorry for my conduct."

Before his plea, Mr. Kruger resigned from the Senate seat he held for 17 years, upsetting the political power dynamic in Brooklyn and complicating Democrats' efforts to retake the chamber next fall. A former Assembly staffer, Mr. Kruger first won office in a 1994 special election and rose to become the influential chairman of the Senate's Finance Committee.

Mr. Kruger's resignation letter, addressed to the secretary of the Senate, simply read: "I hereby resign my office as Senator for the 27th District, King County, effective immediately."

Federal prosecutors accused Mr. Kruger soliciting bribes from two health-care executives, a lobbyist and a real-estate developer in exchange for his key support for their interests. He was ensnared in a wide-ranging influence peddling probe along with seven others, including Assemblyman
William Boyland Jr.

Mr. Kruger pleaded guilty to four of five counts, which include two counts of conspiracy to commit honest services fraud and two counts of conspiracy to commit bribery.

In a statement Tuesday, Preet Bharara, U.S. Attorney for the Southern District of New York, said: "With Senator Kruger's guilty plea today, yet another lawmaker-turned-lawbreaker has now been removed from an undeserved position of power in Albany. Instead of serving the people who elected him, Senator Kruger monetized his public office and served himself."

Mr. Kruger's plea was part of an agreement with prosecutors who recommended that he serve between nine and 11 years in prison. U.S. District Judge Jed Rakoff said he would consider the prosecutors' recommendation but noted that Mr. Kruger faces up to 50 years incarceration for his crimes.

"I'm going to make my own determination regardless of what anyone puts in a letter," Judge Rakoff said, referring to the plea agreement.

A co-defendant,
Michael Turano,
a gynecologist whom prosecutors said Mr. Kruger lived with and whom prosecutors described as an "intimate" associate of Mr. Kruger, was also charged and pleaded guilty Tuesday to one count of conspiracy to commit bribery, which could result in a maximum of five years in prison. Mr. Turano controlled the bank accounts into which the bribes intended for Mr. Kruger went.

Mr. Kruger—free on bail pending his sentencing on April 26—declined to comment. Outside court, his attorney,
Benjamin Brafman,
read a statement to reporters. "By accepting responsibility for his conduct, Sen. Kruger has confirmed his respect for the integrity of the judicial process," Mr. Brafman said. "Mr. Kruger has honestly served during a lifetime of public and community service which, although obviously flawed, is still nevertheless, on balance, quite extraordinary."

The sudden departure of the veteran lawmaker is likely to spark a hotly contested race between Republicans seeking to increase their 32-30 majority and Democrats hoping to hold on to a seat in a district of South Brooklyn that can swing in either direction.

"This closes a sad chapter for the people of Mr. Kruger's district and proves that no one is above the law," Senate Minority Leader
John Sampson
said in a statement. "Restoring faith, trust and confidence in government must be our first priority."